Synergistic Tribological Performance of Phosphorus- and Sulfur-Based Extreme Pressure and Anti-Wear Additives

Abstract

Higher demands on extreme pressure lubrication performance are posed by stringent working conditions. In this study, the synergistic tribological properties of phosphate ammonium salt in combination with active sulfurized olefin (S1) and non-active sulfurized fatty acids (S2) were investigated to meet the needs under stringent working conditions. The anti-wear mechanisms were further explored using scanning electron microscopy (SEM) with EDS, X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), and focused ion beam microscopy. The experimental results indicate that P-S2 demonstrates superior friction reduction and wear resistance under low loads, potentially attributable to its higher polarity, whereas P-S1 exhibits better wear resistance under high loads. P-S1 also shows superior extreme pressure performance attributed to its higher active sulfur content and stronger film-forming ability, evidenced by a thicker friction film (82.62 nm vs. 24.28 nm for P-S2). The study highlights that the variations in the synergistic tribological performance of phosphorus- and sulfur-based additives may link to differences in molecular structure, active sulfur content, polarity, and corrosiveness, with P-S1 demonstrating enhanced extreme pressure performance possibly through the formation of a multi-layered friction film of polyphosphate, sulfide, oligophosphate, and sulfate layers.

1. Introduction

Extreme working conditions in contemporary machinery manufacturing are characterized by high speeds, heavy loads, and elevated temperatures [1]. With the increasing demand for metal materials in the fields of intelligent manufacturing, automobile and steel industries, the electronic information industry, etc., metalworking fluids face significant challenges amid rapid development [2]. For instance, in the steel cold rolling sector, the development of the rolling process and the increasing quality requirements of the market for rolling stainless steel strips have led to the stricter requirements for the performance of rolling oil. These stringent working conditions pose higher demands on the extreme pressure lubrication performance of rolling lubricant fluids. Therefore, it is necessary to research and optimize lubricant systems to meet the needs under stringent working conditions [3,4].

In practice, various types of extreme pressure and anti-wear additives are often combined to enhance lubrication performance [5,6,7]. For example, Kaisei Sato et.al. report that two ionic liquids (ILs) with different cations but similar anions were used in combination with an anti-wear additive, zinc dialkyldithiophosphate (ZDDP), and the results showed that mixed ZDDP and IL solutions exhibited lower friction and wear when compared to ZDDP alone [5]. Ionic liquids, composed of cations and anions, are a class of ionic compounds that remain in a liquid state at room temperature. Due to the negative charge of the anions, they readily adsorb onto freshly exposed friction surfaces, forming an effective boundary lubrication film. Additionally, during the friction process, a portion of the ionic liquid undergoes thermal decomposition, leading to the formation of an anti-wear lubricant film through chemical reactions between non-metal elements and iron and/or oxygen. Phosphate ammonium salt-based ionic liquids have gained significant attention for their combination of superior extreme pressure performance of nitrogen and phosphorus with excellent film-forming capabilities [8,9,10,11,12,13]. For example, Barnhill et.al. studied five quaternary (aprotic) and four tertiary (protic) ammonium ionic liquids (ILs) with an identical organophosphate anion as lubricant anti-wear additives, and the selected ILs were applied as oil additives in steel–cast iron tribological tests and demonstrated promising anti-scuffing and anti-wear functionality [13]. Sulfur-containing extreme pressure additives are particularly effective in high-load applications due to their superior extreme pressure properties and high operating temperature range [14,15]. In this study, we examined the synergistic tribological performance and underlying mechanisms of phosphate amine salt in combination with two typical sulfur-containing extreme pressure agents: active sulfurized olefin (S1, Swt% = 40%) and non-active sulfurized fatty acids (S2, Swt% = 10%).

Biodegradable lubricating materials have been extensively studied [16,17]. Synthetic esters offer superior thermal oxidation stability and biodegradability compared to mineral oils, making them an ideal base oil for environmentally friendly lubricants [17,18,19,20,21].

In this study, we investigated the synergistic tribological properties of phosphate amine salt combined with different sulfur-containing extreme pressure agents in the base oil PETO. Additionally, the anti-wear mechanism was explored using scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), and focused ion beam microscopy.

2. Materials and Methods

2.1. Materials

All additives and the base oil PETO were commercially available, of which the phosphate amine salt was from Gimir Chemical Company (Guangdong, China), the active sulfurized olefin (S1, Swt% = 40%) and non-active sulfurized fatty acids (S2, Swt% = 10%) were from DIC Company, and the PETO was from Nanjing Well Pharmaceutical Technology Co. (Nanjing, China).

2.2. Characterization

2.2.1. Sample Preparation

PETO was used as the base oil in this study. Phosphate amine salt was added at a concentration of 1.0 wt%, along with different sulfur-containing extreme pressure and anti-wear additives. The mixtures were stirred at room temperature for 30 min. The resulting samples were designated as P-S1 and P-S2 as detailed in

Table 1. Composition of lubricants.

Sample	Extreme Pressure and Anti-Wear Additives	Component Ratio
P-S1	S-1: Sulfurized Olefin (Swt% = 40%)	1% Phoshate Amine Salt + 3% S-1 + PETO (balance)
P-S2	S-2: Non-Active Sulfurized Fatty Acids (Swt% = 10%)	1% Phoshate Amine Salt + 5% S-2 + PETO (balance)

.

2.2.2. Testing Instruments

The tribological properties were measured using a four-ball friction testing machine (MS-10J, Xiamen Tenkey Automation Co., Ltd., Xiamen, China). The test conditions for evaluating anti-wear and friction reduction performance were set at 25 °C (room temperature), 1450 r/min, with a test duration of 30 min and loads of 98 N, 196 N, 294 N, 392 N, and 490 N. The material of the steel ball was CG215R. The test parameters for maximum non-seizure load (PB) and sintering load (PD) were 25 °C, 1760 r/min, with a test duration of 10 s. The coefficient of friction (COF) was recorded automatically by the system, and the average value was used. The wear scar diameter (WSD) was measured using an optical microscope with an accuracy of ±0.01 mm.

The morphology of the wear surface and the distribution of active elements were analyzed using a Nano SEM 450 FEI scanning electron microscope (SEM) equipped with a Kevex spectrometer (EDS) from Thermo Fisher, Waltham, MA, USA. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an AXIS UltraDLD instrument provided by Kratos Corporation, Kawasaki, Japan. X-ray absorption near-edge structure (XANES) experiments were performed at the Beijing Synchrotron Radiation Facility (BSRF) 4B7A line station, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China. Additionally, ultra-high-resolution scanning electron microscopy combined with focused ion beam time-of-flight secondary ion mass spectrometry (FIB/SEM, Tescan, Brno, Czech Republic, CAIA3) was used for cross-sectional analysis of friction films formed on the rubbed surfaces. This allowed for the analysis of friction film thickness and the investigation of how molecular structure influences the formation of the friction film.

3. Results and Discussion

3.1. Friction and Wear Behaviors of P-S1/P-S2

The average coefficient of friction (COF) and the wear scar diameter (WSD) of P-S1 and P-S2 under different loads are shown in Figure 1 and Figure 2.

As shown in Figure 1, the average coefficient of friction for P-S2 under different loads was lower, indicating superior anti-wear performance. The COF is primarily associated with the additive’s adsorption capability on the metal surface. In general, higher polarity enhances the adsorption capacity of additives on the steel ball surface. Although the polarity of PETO may limit the movement of the anion–cation pairs and their adsorption on the metal surface, the polarity of the sulfur additives counteracts this effect, improving the adsorption ability of the additives. The polarity of the two sulfur additives ranks as S2 (non-active sulfurized fatty acids) > S1 (sulfurized olefin). Consequently, P-S2 rapidly adsorbs at the friction interface during the friction process, forming an adsorption film.

As shown in Figure 2, the wear scar diameter for P-S2 under the loads of 98 N and 294 N was lower than that of P-S1. However, the wear scar diameter for P-S2 increased rapidly with the increase in the load and was even bigger than that of P-S1 under the load of 490 N, indicating that P-S1 has better anti-wear performance under high loads. It may relate to the higher active sulfur content of P-S1, which produces a high-strength protective film under high loads.

The PB value and PD value for the P-S1 and P-S2 additives are presented in

Table 2. Results of the tribological and extreme pressure tests of the samples.

Samples	PB (kg)	PD (kg)
P-S1	238	450
P-S2	225	238

.

The PB and PD values for P-S1 were higher, which is primarily related to the active sulfur content. A higher active sulfur content enables rapid reaction with the metal surface, producing a high-strength protective film and exhibiting excellent extreme pressure properties. Since S1 has a higher active sulfur content, the PB and PD values of P-S1 are correspondingly higher.

3.2. Morphology of the Worn Surface

3.2.1. SEM with EDS of the Worn Surface

To elucidate the tribological mechanisms of phosphorus–sulfur additives, the microstructure and elemental distribution of the friction surfaces were analyzed using SEM. As shown in Figure 3 and Figure 4, the plow grooves and flaking areas of the wear surfaces of both P-S1 and P-S2 become heavier with the increase in load, which is consistent with the tribological test results. It is worth noting that the worn surface of P-S1 exhibits deeper plow grooves than that of P-S2, especially under the applied loads of 294 N and 490 N, indicating the presence of heavier abrasive wear in P-S1 during the friction behavior. This is ascribed to the higher content of active sulfur in P-S1 (Table 1).

The EDS spectra for C, P, S, N, O, and Fe on the wear surfaces, along with their elemental contents, are presented in Figure 3. The ratios of O/P and O/S for the wear scars lubricated with P-S1 and P-S2 are depicted in

Table 3. Ratios of O/P and O/S for the wear scars lubricated with P-S1 and P-S2.

Samples	O/P	O/S
P-S1 (98 N)	1.086	0.789
P-S1 (196 N)	1.440	1.784
P-S1 (294 N)	2.589	6.51
P-S1 (392 N)	1.530	2.437
P-S1 (490 N)	1.419	1.879
P-S2 (98 N)	1.047	0.717
P-S2 (196 N)	1.143	1.098
P-S2 (294 N)	1.423	1.89
P-S2 (392 N)	1.145	1.611
P-S2 (490 N)	1.098	1.508

.

Phosphorus–oxygen and sulfur–oxygen compounds were detected on the wear surfaces of both samples, influencing their tribological properties. The significant changes in oxygen content could be attributed to the oxidation of the metal substrate surface due to the high local temperatures generated during the friction process or to the formation of tribochemical products with relatively high O/P and O/S ratios.

As shown in Table 3, the O/P and O/S ratios in P-S1 were higher than those in P-S2, which aligned with the previously observed PB and PD values. This may be related to the adsorption and decomposition mechanisms of the additives under tribological heat conditions. The protective film formed may be prone to mechanical scratching due to insufficient strength, where tribochemical products play a crucial role in enhancing tribological performance. Based on the EDS analysis, it can be inferred that wear surfaces with higher O/P and O/S ratios exhibit improved tribological properties. The synergistic effect between phosphate amine salt and the sulfur additive likely contributes significantly to the enhancement of these properties.

The subsequent XPS, XANES, and FIB analyses are based on the four-ball wear scars under a load of 294 N.

3.2.2. XPS Analysis of Worn Surfaces

The worn surfaces of the steel balls lubricated with PETO were analyzed using X-ray photoelectron spectroscopy (XPS), and the XPS spectra were fitted to the curves shown in Figure 5. The O_1s spectrum can be deconvoluted into three peaks at binding energies of 529.6 ± 0.1 eV, 531.8 ± 0.1 eV, and 533.2 ± 0.1 eV, which correspond to Fe-O, bridging oxygen (mainly C-O-C), and non-bridging oxygen or other oxygen-containing groups in the polycarbonate chain, respectively. These findings suggest that the tribochemical film may contain FeCO₃, FeO, and Fe₂O₃.

Figure 6a,b show the XPS spectra of P-S1 and P-S2. Peaks at 707.13 eV, 710.81 eV, 712.23 eV, and 724.44 eV correspond to different iron oxidation states. The presence of peaks at 531.11 eV for O_1s and 133.5 eV for P2p indicates the formation of phosphates and polyphosphates on the wear surfaces of the steel balls. The tribofilm likely contains FeO, Fe₂O₃, FePO₄, and mixed iron phosphates (Fe_n(PO₄)_m). Additionally, the peaks at 160–169 eV for S2p suggest the presence of FeSO₄, FeSO₃, FeS₂O₄, FeS₂, and FeS, indicating that the protective layer is formed through tribochemical reactions involving sulfur and phosphorus additives. This tribochemical layer further enhances the tribological properties of the additives.

3.2.3. Wear Surface XANES Analysis

In XANES (X-ray absorption near-edge structure) spectroscopy, FY (fluorescence yield) and TEY (total electron yield) represent two distinct signal detection modes, each embodying different signal collection methods and emphasizing different types of information. The FY mode, or fluorescence yield, involves analyzing by detecting the intensity of the fluorescent X-rays emitted by the sample after it absorbs X-rays. The advantage of the FY mode lies in its deep penetration depth into the sample, allowing it to detect information from deeper layers within the sample. The TEY mode, or total electron yield, involves analyzing by detecting the total number of electrons emitted by the sample after it absorbs X-rays. The advantage of the TEY mode is its high sensitivity to changes on the sample surface, making it suitable for studying the chemical composition, oxidation states, and chemical bonds of the sample surface. The FY mode and the TEY mode were both used in this paper to achieve a more comprehensive analysis of the sample.

The K-edge XANES spectra of the model compounds are shown in Figure S1, while the K-edge XANES spectra of phosphorus (P) in the FY mode (fluorescence yield mode) with different additives are presented in Figure 6. The K-edge peak positions of phosphorus in the FY mode for various model compounds are listed in

Table 4. K-edge (FY mode) XANES spectra of template compounds and different additives.

Item	FY P K-Edge Peak Position (eV)
P-S1	2152.5
P-S2		2153.8
Model compounds
FePO4		2154.5
Na4P2O7	2151.3	2153.4

. The characteristic peak for polyphosphate is observed between 2150 and 2160 eV, with higher phosphate polymerization shifting the peak position toward lower energy.

As shown in Figure 7 and Table 4, phosphate friction films were formed on the wear surfaces of both additives, consistent with the XPS results. The composition of the friction films formed by the two additives on the metal surface is not fundamentally different; however, the difference in tribological performance is attributed to variations in adsorption and reaction extent. Compared to the spectra of FePO₄ and Na₄P₂O₇, a high-intensity peak at 2152.5 eV was observed on the friction surface of P-S1, indicating the formation of a highly polymerized phosphate reaction film. In contrast, P-S2 formed strong peaks at 2153.8 eV, suggesting the formation of only oligomeric FePO₄ friction films. Therefore, the K-edge XANES absorption spectra of phosphorus in the FY mode for different additive samples reveal a higher amount of polyphosphate on the surface lubricated with P-S1.

Figure 8 displays the K-edge XANES spectra of phosphorus in the TEY mode (total electron yield model) with different additives, with peak positions listed in

Table 5. K-edge XANES absorption spectra of phosphorus in TEY mode.

Item	TEY P K-Edge Peak Position (eV)
P-S1	2150.8	2152.5
P-S2		2155.2
Model compounds
FePO4		2154.5
Na4P2O7	2151.3	2153.4

. Significant differences were observed in the content and degree of polymerization of polyphosphates formed by the two additives. For P-S1, high-intensity peaks were observed at both 2150.8 eV and 2152.5 eV, indicating the formation of a phosphate reaction film with both high polymerization and oligomeric FePO₄ on the surface. In contrast, P-S2 only showed strong peaks at 2155.2 eV, indicating the formation of oligomeric FePO₄ friction films with weak extreme pressure capabilities.

The K-edge XANES spectra of the model compounds (FeSO₄, FeS₂O₄, FeSO₃, FeS, and FeS₂) are shown in Figure S2, with the FY mode spectra for sulfur (S) in different additives shown in Figure 9. The K-edge peak positions for the FY mode spectra for sulfur in several model compounds are listed in

Table 6. K-edge XANES absorption spectra of sulfur in FY mode.

Item	FY S K-Edge Peak Position (eV)
Peaks (103 ev)	a: FeS	b: FeS2	c: FeS2O4	d: FeSO3	e: FeSO4
P-S1	2.4715	/	2.4761	2.4776	2.4828
P-S2	/	/	2.4738	/	2.4820
Model Compounds	2.47	2.472	2.473~2.4815	2.476	2.482

. For P-S1, four reaction compounds were identified in the friction film: FeS, FeS₂O₄, FeSO₃, and FeSO₄. In contrast, no FeS peak was observed in the P-S2 friction film. This indicates that the friction film near the metal surface mainly consists of sulfides, which contribute to an improved PB value. The increase in active sulfur content likely facilitates the formation of sulfides such as iron sulfide or ferrous sulfide near the metal surface.

Figure 10 shows the K-edge XANES spectra of sulfur in the TEY mode with different additives, and the peak positions are listed in

Table 7. K-edge XANES absorption spectra of sulfur in TEY mode.

Item	TEY S K-Edge Peak Position (eV)
Peaks (103 ev)	a: FeS	b: FeS2	c: FeS2O4	d: FeSO3	e: FeSO4
P-S1	/	2.4728	2.47614	/	2.4824
P-S2		2.4724	/	/	2.4849
Model compounds	2.47	2.472	2.473~2.4815	2.476	2.482

. Both P-S1 and P-S2 friction films exhibited characteristic peaks of sulfides and sulfates, primarily FeS₂, FeS₂O₄, FeSO₃, and FeSO₄, which aligns with the XPS analysis. This indicates that the friction film farther from the metal surface mainly consists of oxysulfides.

3.2.4. Analysis of FIB-Cut Friction Films on Worn Surfaces

To visualize the thickness and chemical composition of the friction films formed by the additive molecules during the friction process, focused ion beam (FIB) cutting was performed on selected regions of the friction area. The cross-sections of these cut friction films were then characterized using SEM and EDS to observe the morphology and elemental distribution, as shown in Figure 11. The SEM images revealed a distinct boundary between the metal substrate and the friction film on the lubricated surface. The measured thickness of the friction film after P-S1 treatment was 82.62 nm, while the thickness after P-S2 treatment was 24.28 nm. The P and S elements can also be observed on cross-sectional samples by EDS, which further confirms the formation of phosphorus- and sulfur-containing tribofilms that can effectively protect the metal surface from further wear. The ranking of friction film thickness was P-S1 > P-S2, which correlates with the higher active sulfur content in P-S1, leading to a stronger film-forming ability and a thicker film, consistent with the observed PB and PD values.

3.3. Tribological Mechanism Analysis of Phosphorus and Sulfur Extreme Pressure Antiwear Agents

Based on the tribological properties and SEM-EDS, XPS, XANES, and FIB analyses discussed above, the mechanism by which phosphorus- and sulfur-containing extreme pressure and anti-wear additives function on the friction surface appears to be related to the decomposition processes of phosphate esters and sulfur additives.

Phosphate amine salt, an ionic liquid, initially adsorbs onto the metal surface due to surface charge attraction, outcompeting sulfur additives. The polarity ranking of the sulfur additives is S2 (fatty acids) > S1 (olefin), while the sulfur and active sulfur content ranking is S1 (40%) > S2 (10%).

The proposed sequence of friction film formation is as follows: first, phosphate amine salt adsorbs and decomposes on the metal surface due to its high reactivity, containing both phosphorus and nitrogen elements. This makes it more reactive with micro-asperities. Next, due to competitive adsorption, the higher polarity S2 additive also adsorbs on the surface, forming sulfides and sulfates through tribochemical reactions. The lower sulfur content and active sulfur in S2 contribute to its superior friction reduction and wear resistance.

In contrast, S1, with stronger sulfide formation ability, reacts more readily near the metal surface. However, due to its relatively lower polarity, S1 does not significantly interfere with the formation of the phosphate friction film and may even protect its formation. The sulfates formed are located farther from the metal surface. Therefore, the friction film for P-S1 may consist of polyphosphate, sulfide, oligophosphate, and sulfate layers. The multi-layered, thicker friction film formed by P-S1 results in superior extreme pressure performance.

While this study provides valuable insights into the friction properties and mechanisms of two different sulfur-containing extreme pressure additives combined with ammonium phosphate salt in PETO, it is acknowledged that the findings are based on the specific structure of extreme pressure additives tested under particular conditions and load ranges. This limits the generalizability of the results to other additive structures. Future research will aim to extend the scope of extreme pressure additives to explore additional structures and investigate the performance limits of these additives under a broader range of operational conditions.

4. Conclusions

In this study, the friction properties and mechanisms of two different sulfur-containing extreme pressure additives combined with ammonium phosphate salt in PETO were investigated. The main conclusions are as follows:

(1)

The COF and WSD of P-S2 was lower than that of P-S1 under low loads. Due to its higher polarity, P-S2 exhibits better friction reduction and wear resistance performance. However, the wear scar diameter for P-S2 increased rapidly with the increase in the load and is even bigger than that of P-S1 under the load of 490 N, indicating that P-S1 has better anti-wear performance under high loads.

(2)

The values of PB and PD are P-S1 > P-S2. Mainly due to its higher active sulfur content, P-S1 exhibits better extreme pressure performance.

(3)

The thickness of the friction films formed at the friction interface was ranked P-S1 (82.62 nm) > P-S2 (24.28 nm), indicating that P-S1 has a stronger film-forming ability, consistent with the PB and PD values.

(4)

The friction film on the surface likely includes polyphosphate, sulfide, oligo-phosphate, and sulfate, arranged from near to far from the metal surface. The formation of a multi-layered, thicker friction film with more polyphosphate allows P-S1 to exhibit superior extreme pressure performance.